Drug delivery devices allowing for multiple dosing of a required dosage of a liquid medicament, such as liquid drugs, and further providing administration of the medicament to a patient, are as such well-known in the art. Generally, such devices have substantially the same purpose as that of an ordinary syringe.
Drug delivery devices of this kind have to meet a number of user specific requirements. For instance in case of those with diabetes, many users will be physically infirm and may also have impaired vision. Therefore, these devices need to be robust in construction, yet easy to use, both in terms of the manipulation of the parts and understanding by a user of its operation. Further, the dose setting must be easy and unambiguous and where the device is to be disposable rather than reusable, the device should be inexpensive to manufacture and easy to dispose. In order to meet these requirements, the number of parts and steps required to assemble the device and an overall number of material types the device is made from have to be kept to a minimum.
With drug delivery devices such as pen-type injectors, the medicament to be dispensed is provided in a cartridge usually comprising a vitreous body. Said body is typically sealed in proximal direction with a displaceable piston. FIG. 1 is illustrative of such a pen-type injector 10 having a drive mechanism 12 operable to exert distally directed thrust to a displaceable piston 28 of a cartridge 26 mounted therein. As shown in FIG. 1, a housing component 20 of the drive mechanism 12 comprises a radially inwardly extending guiding member or web 22 having an inner thread 24 that cooperates and mates with an external thread of a piston rod 14. The piston rod 14 is to be driven either manually or in an automated way. A rotation of the piston 14 leads to a displacement of the piston in distal direction 34. In the course of such distal displacement, a bearing disc 18 rotatably mounted on a distal bearing 16 of the piston rod 14 buts against a pressure-receiving surface of the piston 28.
Since the piston 28 is movably disposed between the sidewalls 26 of the cartridge, actuation of the piston rod 14 therefore leads to a distally directed displacement of the piston 28, thereby expelling a pre-defined amount of a liquid medicament from a distal end of the cartridge not further illustrated here. The cartridge 26 itself is accommodated in a cartridge holder 30 featuring a proximal insert piece 32, by way of which cartridge holder 30 and housing 20 can be mutually interconnected.
During a dose dispensing action, the piston 28 has to be displaced in distal direction 34 with respect to the body 26 of the cartridge. Respective counter acting or holding forces are provided by the interconnected housing components 30, 20 of the drug delivery device. Moreover, the piston 28 typically comprises an elastic rubber material, inherently exhibiting a rather slow relaxation in response to mechanical compression. After dispensing of a dose of the medicament, a non-negligible fluid pressure may therefore build up inside the body 26 of the cartridge due to an elastic relaxation of the piston and/or of a distally located piercable seal.
Furthermore, since various components of the housing 20, 30 of the drug delivery device 10 as well as of the drive mechanism 12 are manufactured of thermoplastic materials, the device and its drive mechanism may feature a certain axial play or clearance. Internal mechanical stress of various device components as well as relaxation processes of resilient or deformable components of the device and/or its cartridge may lead to a substantial pressure built up inside the cartridge, which may effectuate a post-dispensing droplet generation at the distal tip of an injection needle being in fluid communication with the inner volume of the cartridge.
The problem of such droplet generation in medical delivery devices is already addressed in WO 2009/146996 A1. There, a pressure relief mechanism is “built in” in the design of a plunger rod. Said pressure relief mechanism comprises at least one alteration in the pitch of the threads of a plunger rod arranged to interact with a number of protrusions around the inner circumferential surface in a nut for releasing the pressure exerted upon a stopper by the relative rotational movement between plunger rod and nut. Hence, when the plunger rod has rotated almost a full distance as set by a driver member, protrusions of the nut reach the end of an increased pitch of the threads of the plunger rod and fall into the sudden alteration of the pitch. Such movement causes a pressure relief of the plunger rod and therefore also a pressure relief of the content of the container.
However, such a pressure relief mechanism may increase axial clearance between drive mechanism and piston at the expense of dosing and dispensing accuracy.
Furthermore and irrespective of such a pressure relief mechanism, the general problem may arise, that due to internal pressure built up inside the cartridge and/or due to elastic relaxation of the piston or seal of the cartridge, the piston itself may exert a proximally directed pressure to the piston rod. Hence, in particular circumstances, the elastically relaxing piston of the cartridge may induce a reverse and a proximally directed displacement of the piston rod as soon as a dispensing procedure has been completed. Due to unavoidable manufacturing and assembly tolerances, such a reverse motion is disadvantageous in terms of dosing and dispensing accuracy.
Also, the mechanical components of the drive mechanism may become subject to malfunction. In fact, a proximally directed displacement of a piston rod may cause a jam of the drive mechanism and may therefore lead to severe problems for following dose setting and dispensing procedures.
It is therefore an object of the present invention to reduce or to entirely eliminate proximally directed mechanical stress acting on a piston rod of an injection device's drive mechanism. It is a further object to reduce the drive mechanisms susceptibility to errors and malfunction and to increase dosing and injection accuracy of the device. Moreover, the invention aims to improve patient safety, in particular for disposable drug delivery devices.